The present invention relates to golf balls and, in particular, to golf balls having at least one layer comprising a blend of at least one saponified polymeric material and at least one polyolefin polymer produced using a single-site metallocene catalyst in the polymerization process. The metallocene catalyzed polymer may be unmodified, or may contain at least one pendant functional group that is grafted to the polymer chain by a post-polymerization reaction. The layer, which may be foamed or unfoamed, may be located in any of the cover or core of the ball or in a mantle layer located between the cover and the core.
Three-piece, wound golf balls with balata covers are preferred by most expert golfers. These balls provide a combination of distance, high spin rate, and control that is not available with other types of golf balls. However, balata is easily damaged in normal play, and, thus, lacks the durability required by the average golfer.
In contrast, amateur golfers typically prefer a solid, two-piece ball with an ionomer cover, which provides a combination of distance and durability. Because of the hard ionomer cover, these balls are almost impossible to cut, but also have a very hard xe2x80x9cfeelxe2x80x9d, which may golfers find unacceptable, and a lower spin rate, making these balls more difficult to draw or fade. The differences in the spin rate can be attributed to the differences in the composition and construction of both the cover and the core.
Many attempts have been made to produce a golf ball with the control and feel of a wound balata ball and the durability of a solid, two-piece ball, but none have succeeded totally. In various attempts to produce an ideal golf ball, the golfing industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 to 66 on the Shore D scale, as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been unsatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
While different blend combinations of species of one variety of polymer, such as prior art ionomers, i.e., copolymers of an olefin and an xcex1,xcex2-unsaturated carboxylic acid, have been successfully used in the prior art, different polymers, such as carboxylic acid based ionomers and balata or other non-ionic polymers have not been successfully blended for use in golf ball covers. In general, prior art blends of polymer components are immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture, such as those observed between carboxylic acid based ionomers and other polymers containing carboxylic acid groups. In particular, this lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability, cut resistance, and so on. In contrast, a compatible blend may be heterogeneous on a microscopic scale, but is homogeneous on a macroscopic scale, and, thus, has useful golf ball properties.
In this regard, U.S. Pat. No. 5,397,840 discloses golf ball covers including a blend of xe2x80x9cionic copolymersxe2x80x9d and xe2x80x9cnon-ionic copolymersxe2x80x9d. However, the xe2x80x9cionic copolymersxe2x80x9d are defined as copolymers of an xcex1-olefin and a metal salt of an xcex1xcex2-unsaturated carboxylic acid, and the xe2x80x9cnon-ionic copolymersxe2x80x9d are copolymers or terpolymers containing ethylene or propylene and acrylic or methacrylic acid monomers. Therefore, strong interactions exist between the metal salts of the xe2x80x9cionic copolymersxe2x80x9d and the acrylic or methacrylic acid monomers of the xe2x80x9cnon-ionic copolymersxe2x80x9d that allow compatible blends to be formed. These interactions do not exist in prior art blends of ionomers and polymers that are truly non-ionic or nonpolar, in particular, those polymers produced with a process that does not involve the use of a metallocene catalyst.
The use of single-site metallocene catalysts in the polymerization of polyolefins produces polymers with a narrow molecular weight distribution and uniform molecular architecture, so that the order and orientation of the monomers in the polymer, and the amount and type of branching is essentially the same in each polymer chain. The narrow molecular weight distribution and uniform molecular architecture provides metallocene polymers with properties that are not available with conventional polymers, and allow polymers to be produced having unique properties that are specifically tailored to a particular application. The desired molecular weight distribution and the molecular architecture are obtained by the selection of the appropriate metallocene catalyst and polymerization conditions. The properties may than be further tailored to an application by grafting an appropriate functional group to the polymer chain using a post-polymerization reaction.
Grafted metallocene catalyzed polymers, which are available commercially, share the advantages of unmodified metallocene catalyzed polymers, including a narrow molecular weight distribution and uniform molecular architecture. The addition of functional groups to the metallocene catalyzed polymers by grafting allows polymers to be produced having properties that are not available with unfunctionalized metallocene catalyzed polymers or polymers formed without the use of metallocene catalysts.
As shown in co-pending patent application Ser. No. 08/482,514, metallocene catalyzed polymers and ionomers form compatible blends having useful golf ball properties. However, there is no known disclosure of golf balls comprising compatible blends of grafted or non-grafted metallocene catalyzed polymers, i.e., polymers produced using single-site metallocene catalysts, and ester based ionomeric polymers produced by carrying out a hydrolysis or saponification on copolymers containing pendant ester groups to form an ionomeric polymer that is less hydrophilic than typical carboxylic acid based ionomers.
Hydrolysis or saponification of alkyl acrylate units in a crosslinkable polymer chain is disclosed by Gross in U.S. Pat. No. 3,926,891. This is accomplished by dissolving the polymer in an aqueous alkali metal hydroxide solution and then heating. The product is recovered by coating the solution onto a substrate and evaporating the water or by extruding the solution into a non-solvent. In U.S. Pat. No. 3,970,626, Hurst discloses heating a mixture of an alkali metal hydroxide, a thermoplastic ethylene-alkyl acrylate copolymer and water to saponify the acrylate units and form an aqueous emulsion. This emulsion can be used as such, partially dried to a paste or moist solid, or fully dried to solid form.
A different approach to hydrolysis or saponification of an ethylene-alkyl acrylate copolymer is disclosed by Kurkov in U.S. Pat. No. 5,218,057, in which the copolymer is mixed with an aqueous solution of an inorganic alkali metal base at a temperature sufficient for saponification to take place and at which the copolymer undergoes a phase change. Typically, the copolymer would be molten when mixed with the aqueous solution.
All of these prior methods require that the polymer component be in contact with water, either by conducting the reaction in an aqueous medium or by adding an aqueous solution to the polymer. Processes of this nature pose several disadvantages, however. First, it is difficult to remove water from the hydrolyzed or saponified polymer product. The polymer product is in the form of a salt that has a more polar nature than the reactant acrylate ester, and so is more likely to associate with or hydrogen bond to a polar solvent like water. The energy required to remove a highly interacting polar solvent like water is much greater than for a nonpolar or weakly polar organic solvent. Second, it is important to remove water from the ionomer product because the presence of water can have detrimental effects on ionomer mechanical properties imparted by the polar ionic domains, which act as the effective crosslink sites. Residual water weakens the ionic interactions within these domains, thereby reducing the mechanical property benefits the domains impart. Finally, incomplete removal of water can lead to difficulty in later fabricating steps where the product ionomer is reheated and shaped, e.g., into golf ball covers. Residual water can cause undesirable irregularities and imperfections on the surface of fabricated articles by the formation of blisters. Residual water within fabricated polymer articles can lead to void formation and even uncontrolled foaming with a concomitant undesirable influence on the mechanical properties, load bearing capacity and durability of the fabricated articles.
Melt state neutralization of an ethylene-acrylic acid copolymer by a solid, solution or slurry of an alkali metal salt is disclosed by Walter in U.S. Pat. No. 3,472,825. In the examples provided, hydrolysis is accomplished by mixing an alkali hydroxide with copolymer at constant temperature either in a Banbury mixer or on a two roll mill. Walter does not disclose the use of extrusion type polymer processing apparatus for this neutralization.
McClain, in U.S. Pat. No. 4,638,034, discloses a process whereby ethylene-acrylic acid copolymers or their ionomers are prepared from ethylene-alkyl acrylate copolymers by saponifying the latter in the melt with metal hydroxides to form an ionomer and a by-product, i.e., alkanol, then optionally acidifying the ionomer to form the free acid copolymer. This process proceeds in the molten state and in the absence of solvent or water, other than the by-product alkanol. Saponification proceeds under non-static mixing conditions, typically with equipment commonly employed in the art of mixing molten polymer materials such as multiroll mills, a Banbury mixer or a twin screw extruder.
The process disclosed by the ""034 reference is, however, incapable of providing optimal product quality since blending and saponifying in a single operation as taught by the subject reference leads to rapid hydrolysis, with a concurrent rapid increase in viscosity. Due to this rapid increase in viscosity, the resultant mixture is non-uniform and therefore the physical properties of products made from this material are not consistent throughout the product.
During the melt state conversion of the alkyl-acrylate copolymer to the metal acrylate copolymer salt, a great decrease in melt flow rate occurs with a corresponding great increase in melt viscosity. While not wishing to be bound by any particular theory, this decreased melt flow rate is thought to occur because of the tendency of the relatively polar ionic salt functionalities formed during the saponification reaction to associate with themselves rather than the relatively nonpolar unreacted alkyl acrylate or comonomer chain segments. Aggregations of salt moieties arising from side groups attached to different chains into ionic domains introduces effective crosslink points throughout the molten copolymer. The effective crosslinks, in turn, greatly increase the copolymer melt viscosity and, correspondingly, greatly decrease copolymer melt flow rate.
A need exists in the golf ball art for highly durable golf balls, which have improved performance, and may be tailored to have virtually any combination of feel and spin rate. The present invention provides such a golf ball.
The present invention is directed to a golf ball having least one layer, where the layer formed of a saponified polymer/metallocene catalyzed polymer blend, comprising from about 1 to about 99 parts of at least one saponified polymer and from about 99 to 1 parts of at least one metallocene catalyzed polymer, based on 100 parts by weight of the saponified polymer/metallocene catalyzed polymer blend. The layer may be foamed or unfoamed, and may form at least a portion of any of the cover, the core, or a mantle layer situated between the cover and the core.
Preferred metallocene catalyzed polymers include olefinic homopolymers, such as polyethylene, and copolymers of ethylene with propylene, butene, hexene, octene, and norbornene, and olefinic homopolymers and copolymers of propylene with butene, hexene, octene, and norbornene. However, the metallocene catalyzed polymer may be any metallocene catalyzed polymer of the formula:
wherein
R1 is hydrogen;
R2 is hydrogen or lower alkyl selected from the group consisting of CH3, C2H5, C3H7, C4H9, and C5H11;
R3 is hydrogen or lower alkyl selected from the group consisting of CH3, C2H5, C3H7, C4H9, and C5H11; 
R4 is selected from the group consisting of H, CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, C9H19, C10H21, and phenyl, in which from 0 to 5 H within R4 can be replaced by substituents selected from the group consisting of COOH, SO3H, NH2, F, Cl, Br, I, OH, SH, silicone, lower alkyl esters and lower alkyl ethers, with the proviso that R3 and R4 can be combined to form a bicyclic ring;
R5 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic;
R6 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic; and
wherein x ranges from 99 to 50 weight per cent of the polymer, y ranges from 1 to 50 weight per cent of the polymer and z ranges from 0 to 49 weight per cent of the polymer.
The saponified polymer typically comprises a first olefinic, monomeric component having from 2 to 8 carbon atoms and a second monomeric component comprising an unsaturated carboxylic acid based acrylate class ester having from 4 to 22 carbon atoms and at least one ester group, wherein at least a portion of the ester groups have been saponified with an inorganic metal base. Useful inorganic metal bases include, but are not limited to metal bases comprising at least one metallic cation, such as lithium, sodium, potassium, cesium, magnesium, calcium, barium, manganese, copper, zinc, aluminum, titanium, tungsten, zirconium, platinum, rubidium, and strontium, and at least one anion, such as hydroxide, alkoxide, acetate, carbonate, bicarbonate, oxide, formate, and nitrate.
Typically, the first monomeric component is an xcex1-olefin monomer having a terminal point of unsaturation, and may be of the formula: 
where R7 is hydrogen or an alkyl group, and R8 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic. Preferably, the first monomeric component is ethylene.
Typically, the first monomeric component comprises from about 1 to about 99 percent by weight of the total polymer weight, preferably from about 10 to about 95 percent by weight of the total polymer weight, and most preferably from about 10 to about 70 percent by weight of the total polymer weight.
The second monomeric component is typically an unsaturated acrylate class ester having the formula: 
where R9 is hydrogen or an alkyl group; R10 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic; R11 is selected from the group consisting of CnH2n+1, for n=1 to 18 and phenyl, in which from 0 to 5 H within R10 can be replaced by substituents selected from the group consisting of COOH, SO3H, NH3, succinic anhydride and their salts, F, Cl, Br, I, OH, SH, epoxy, silicone, lower alkyl esters, lower alkyl ethers, and aromatic or heterocyclic rings with the proviso that R10 and R11 can be combined to form a bicyclic ring. Typically, the second monomeric component comprises from about 99 to about 1 percent by weight of the total polymer weight, preferably, from about 90 to about 5 percent by weight of the total polymer weight, and most preferably, from about 90 to about 30 percent by weight of the total polymer weight.
The saponified polymer may also comprise a third monomeric component. Useful third monomeric components include carbon monoxide, sulfur dioxide, an anhydride monomer, an unsaturated monocarboxylic acid, an olefin having from 2 to 8 carbon atoms and a vinyl ester or a vinyl ether of an alkyl acid having from 4 to 21 carbon atoms. Preferred third monomeric components include monomers of formula 
wherein:
R12 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic;
R13 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic;
R14 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic;
R15 is hydrogen or lower alkyl including C1-C5;
R16 is hydrogen, or is selected from the group consisting of CRH2n+1, for n=1 to 18 and phenyl, in which from 0 to 5 H within R16 can be replaced by substituents selected from the group consisting of COOH, SO3H, NH3 and their salts, F, Cl, Br, I, OH, SH, silicon, lower alkyl esters, lower alkyl ethers and aromatic or heterocyclic rings with the proviso that R15 and R16 can be combined to form a bicyclic ring;
R17 is hydrogen, lower alkyl including C1-C5, carbocyclic, aromatic or heterocyclic;
R18 is hydrogen or lower alkyl including C1-C5;
R19 is hydrogen, or is selected from the group consisting of CRH2+1, for n=1 to 18 and phenyl, in which from 0 to 5 H within R19 can be replaced by substituents selected from the group consisting of COOH, SO3H, NH3 and their salts, F, Cl, Br, I, OH, SH, epoxy, silicon, lower alkyl esters, lower alkyl ethers and aromatic or heterocyclic rings with the proviso that R18 and R19 can be combined to form a bicyclic ring.
Typically, the third monomeric component comprises from about 0 to 49 percent by weight of the total polymer eight of the saponified polymer. The monomeric components of the polymer may be present in a random, alternating, block or graft arrangement, and the saponified polymers may be isotactic, syndiotactic, atactic polymers, or a combination thereof.
A grafting agent may also be added to at least one of the saponified polymer or the metallocene catalyzed polymer to form a grafted polymer. The preferred grafting agent is an anhydride having the formula: 
where R20 and R21 are the same or different, and are typically hydrogen, linear or branched chain alkyl, or substituted or unsubstituted carboxylic groups.
The grafting agent is typically added in an amount of between about 1 to about 50 percent by weight, preferably from about 1 to about 25 percent by weight, and most preferably from about 1 to about 15 percent by weight of the polymer.
The present invention is also directed to a process for forming a golf ball, which comprises forming a polymer comprising a first olefinic monomeric component having from 2 to 8 carbon atoms and a second monomeric component comprising an unsaturated carboxylic acid based acrylate class ester having from 4 to 22 carbon atoms; applying a sufficient amount of heat to the polymer to convert the polymer to a substantially molten state; forming a mixture by adding an inorganic metal base to the molten polymer, such that the viscosity of the mixture remains substantially unchanged from the viscosity of the molten polymer; saponifying the mixture to form a saponified polymer, where a sufficient amount of the inorganic metal base is added to the molten polymer in forming the mixture to obtain a degree of saponification of the polymer ranging between about 1 and 50 percent. The saponified polymer is then blended with a metallocene catalyzed polymer to form a saponified polymer/metallocene catalyzed polymer blend, which is used to form at least one layer of a golf ball. Optionally, the polymer further comprises a third monomeric component, such as carbon monoxide, sulfur dioxide, an anhydride monomer, an unsaturated monocarboxylic acid, an olefin having from 2 to 8 carbon atoms, or a vinyl ester or a vinyl ether of an alkyl acid having from 4 to 21 carbon atoms.